Martensitic steel with mixed hardening

ABSTRACT

The invention relates to a method of producing a martensitic steel comprising a content of other metals such that it can be hardened by intermetallic compound and carbide precipitation, with an Al content of between 0.4% and 3%. The heat shaping temperature of the last heat shaping pass of said steel is lower than the solubility temperature of the aluminum nitrides in the steel, and the treatment temperature for each potential heat treatment after said last heat shaping pass is lower than the solid-state solubility temperature of the aluminum nitrides in said steel.

This application is a division of U.S. application Ser. No. 13/382,045filed Feb. 1, 2012 now U.S. Pat. No. 8,702,879, the entire contents ofwhich is incorporated herein by reference. U.S. application Ser. No.13/382,045 is a National Stage of PCT/FR10/051400 filed Jul. 2, 2010,and is based upon and claims the benefit of priority from prior FrenchApplication No. 0954576 filed Jul. 3, 2009.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for producing martensiticsteel that comprises a content of other metals such that the steel canbe hardened by intermetallic compound and carbide precipitation, with anAl content of between 0.4% and 3%.

DESCRIPTION OF THE RELATED ART

For certain applications, in particular for airplane engine transmissionshafts, it is necessary to use steel having a very high mechanicalstrength (yield strength) up to 400° C., and at the same time goodresistance to brittle fracture (high stiffness and ductility). Thesesteels must also have good fatigue behavior.

One such martensitic steel is known from document U.S. Pat. No.5,393,488, which includes a content of other metals such that it iscapable of being hardened by an intermetallic compound and carbideprecipitation. The composition of such a steel by weight is as follows:10 to 18% of Ni, 8 to 16% of Co, 1 to 5% of Mo, 0.5 to 1.3% of Al, 1 to3% of Cr, less than 0.3% of C, less than 0.1% of Ti, the rest being Fe.

The drawback of such a steel is its high cost, due to its significanceCo content.

Also known as another martensitic steel that comprises the contents ofother metals such that it is capable of being hardened by anintermetallic compound and carbide precipitation, the composition ofwhich is given in document FR 2,885,142 as follows (percentages byweight): 0.18 to 0.3% of C, 5 to 7% of Co, 2 to 5% of Cr, 1 to 2% of Al,1 to 4% of Mo+W/2, traces to 0.3% of V, traces to 0.1% of Nb, traces to50 ppm of B, 10.5 to 15% of Ni with Ni≧7+3.5 Al, traces to 0.4% of Si,traces to 0.4% of Mn, traces to 500 ppm of Ca, traces to 500 ppm of Rareearths, traces to 500 ppm of Ti, traces to 50 ppm of O (development frommolten metal) or to 200 ppm of O (development through powdermetallurgy), traces to 100 ppm of N, traces to 50 ppm of S, traces to 1%of Cu, traces to 200 ppm of P, the rest being Fe.

This steel FR 2,885,142 has a very high mechanical strength (breakingload able to go from 2000 MPa to 2500 MPa) and at the same time verygood resilience (greater than 180·10³ J/m²) as well as a good compromisewith the other properties of toughness and fatigue behavior.

However, the results of fatigue tests conducted on this type of steel bythe inventors show great dispersion in the bench life values(corresponding to the number of cycles leading to the break of a fatiguetest piece in said steel) for each imposed deformation stress level,whether for low-cycle fatigue (stress frequency in the vicinity of 1 Hz)or vibrational fatigue (greater than 50 Hz). Thus, the minimum values,within the statistical meaning, of the fatigue bench life (limiting thebench life of parts made from this steel) are still too low.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to resolve these drawbacks.

The invention aims to propose a method for producing this type of steelthat makes it possible to reduce the fatigue behavior dispersion of thistype of steel, and to increase its average fatigue behavior value.

This aim is achieved owing to the fact that the method is such that theheat shaping temperature for the last heat shaping pass is less than thesolubility temperature of the aluminum nitrides in said steel, and theheat treatment temperature for each optional heat treatment after saidlast heat shaping pass is less than the solid-state solubilitytemperature of the aluminum nitrides in said steel.

In this way, after the last heat shaping pass (for example, forging),the number of unwanted aluminum nitride precipitates per surface unit ofsteel in the form of tabs (or needles) is statistically negligible, andcan be estimated at less than 10⁻¹² units per mm².

In fact, the inventors have noted that among the precipitates ofaluminum nitrides (AlN), it is those in the form of tabs (or needles)that are undesirable, as they act as strain concentrating sites whenthis steel is subjected to fatigue stresses, and thereby reduce theenergy necessary for spreading cracks to start. The inventors have alsonoted that unwanted AlN precipitates form when the aluminum and nitrogenrecombine during cooling from a temperature higher than the solid-statesolubility temperature of the AlN precipitates.

Owing to the method according to the invention, unwanted AlNprecipitates do not form during the last heat shaping pass (for example,forging), since the latter is done at a temperature lower than thesolid-state solubility temperature of said precipitates. Furthermore,any unwanted AlN precipitates present in the steel before this last heatshaping pass (formed during earlier operations that may have been doneat a temperature higher than the solid-state solubility temperature ofthe AlN) are broken by said last heat shaping pass into pieces whereofthe dimensions are of the same order of magnitude in the three spatialdirections, and that are spaced apart from one another. These pieces arethus not very capable of being sites for cracks to begin that would leadto premature ruin of the steel.

As a result, the proportion of unwanted AlN precipitates (precipitatesin the form of tabs or needles) at the end of the last heat shaping passis negligible, such that these precipitates can no longer serve asstarting sites for cracks. Furthermore, these unwanted AlN precipitatesdo not reform after the last heat shaping pass, since the steel nolonger goes above the solid-state solubility temperature of the AlNduring any later heat treatments. This therefore results in an increasein the minimum fatigue bench life values, as well as the average of thefatigue bench life durations.

The invention also relates to a metal steel comprising a content ofother metals such that it is capable of being hardened by anintermetallic compound and carbide precipitation, with an Al content ofbetween 0.4% and 3%.

According to the invention, if the last heat shaping pass is done belowthe solid-state solubility temperature of the aluminum nitrides and thetreatment temperature for each optional heat treatment after said lastheat shaping pass is less than the solid-state solubility temperature,the number of these precipitates having an unwanted shape (tabs orneedles) per surface unit of the steel is statistically less than 10⁻¹².As a result, the dispersion of the results in number of fatigue cycleswill be reduced, resulting in a longer lifetime of a piece made fromsaid steel.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be better understood and its advantages will betterappear upon reading the following detailed description of one embodimentshown as a non-limiting example. The description is done in reference tothe appended drawings, in which:

FIG. 1 compares fatigue bench life curves for a steel according to theinvention and a steel according to the prior art,

FIG. 2 shows a fatigue stress curve,

FIG. 3 is a scanning electron microscope photograph of a secondaryprecipitate in a steel according to the prior art,

FIG. 4 is a scanning electron microscope photograph of a primaryprecipitate in a steel according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

We will consider martensitic steels with mixed hardening with a contentof Al of between 0.4% and 3%. It is their content of Al and of othermetals that allows these steels to be hardened by an intermetalliccompound and carbide precipitation (mixed hardening).

Fatigue tests on test pieces of these steels produced according to theprior art have shown great dispersion in the results of those tests,i.e. for a given fatigue stress strain, the bench life varies over awide range.

The width of this range, specifically the low values of this range, isdue to the presence of these unwanted precipitates that require lessenergy to start fatigue cracks, and leads to premature breaking of thesteel.

Due to its chemical composition, the steel contains primary precipitatesof AlN that form during solidification of the metal, at a hightemperature, when this steel is still in the paste phase (i.e. in thetwo mixed solid and liquid states of the steel). Their quantity is below10⁻² units per mm². This primary precipitation occurs in the form ofprecipitates whereof the dimensions are of the same order of magnitudein all three spatial directions (i.e. these precipitates have asubstantially spherical shape), and the size of which does not exceed 50μm. The beginning of cracks from these primary AlN precipitates requiresmore energy (than from the secondary precipitates, see below) andtherefore does not generate minimal fatigue bench life values.

The inventors noticed that the steel according to the prior art alsocontains AlN precipitates distinct from the primary precipitates, whichare called secondary precipitates. These secondary precipitates arepresent in a lower proportion (less than 10⁻² units per mm²) than theprimary precipitates. It is these secondary precipitates that areresponsible for premature breaking of the steel. The inventors notedthat the secondary precipitates have an unwanted tab (or needle) shapecharacterized by their dimensions in all three directions:

a) the smallest dimension being smaller than one twentieth of thelargest dimension, and

b) the largest dimension being larger than 10 μm.

These secondary precipitates act as strain concentrating sites, andtherefore as favored locations for cracks to start, since cracks requireless energy to form on these secondary precipitates than on the primaryprecipitates. Cracks therefore form prematurely on the secondaryprecipitates, and lead to a decrease in the bench life of the steelpiece (which corresponds to the low values of the test result range).

The inventors have also noted that in solid steel, a second solubilitytemperature exists for the aluminum nitrides AlN (depending on thechemical composition), called solid-state solubility temperature, andthat the secondary AlN precipitates form during cooling of the steelfrom a temperature higher than said solid-state solubility temperatureof the AlN precipitates in the steel. In fact, when the steel goes abovethe solubility temperature, a low proportion of the primary AlNprecipitates dissolve. Then, when the temperature goes back below thistemperature during cooling of the steel, the dissolved aluminum andnitrogen recombine into secondary AlN precipitates.

For example, this solid-state solubility temperature is equal to 1025°C. in the case of a steel whereof the composition is covered by patentFR 2,885,142 and is provided above.

Thus, if the forging temperature of the last heat shaping pass is lowerthan the solid-state solubility temperature of the AlN precipitates inthe solid steel, then the AlN precipitates remain in that state (thealuminum and nitrogen do not dissolve). Secondary AlN precipitatestherefore do not form.

Furthermore, any secondary AlN precipitates that may be present beforethe last forging pass (which may result from prior heat treatments doneabove the solid-state solubility temperature) tend on the one hand to bebroken by the forging operation into smaller precipitates, the shape ofwhich has equivalent dimensions in all three spatial directions (unlikean unwanted shape (tab or needle)), and on the other hand to be spacedapart from one another. Consequently, the heat shaping operation belowwith the solid-state solubility temperature of the AlN precipitatestransforms the harmful secondary precipitates into precipitates moresimilar to primary precipitates, and which are therefore less harmfulfor the fatigue bench life of the steel.

The inventors have performed tests on steels produced using the methodaccording to the invention, i.e. with a last heat shaping passtemperature lower than the solid-state solubility temperature of the AlNprecipitates in the steel (and no subsequent heat treatment above thattemperature), and the results of those tests are presented below.

FIG. 1 qualitatively shows the improvements resulting from the methodaccording to the invention. One determines the value of the number N ofbreak cycles necessary to break a steel test piece subject to a cyclicalpulling stress as a function of the pseudo-alternating stress C (this isthe stress undergone by the test piece under an imposed deformation,according to Snecma standard DMC0401 used for these tests).

One such cyclical stress is diagrammatically illustrated in FIG. 2. Theperiod T represents one cycle. The strain evolves between a maximumvalue C_(max) and a minimum value C_(min).

By fatigue testing a statistically sufficient number of test pieces, theinventors obtained points N=f(C) from which they drew a mean statisticalcurve C-N (strain C_(max) as a function of the number N of fatiguecycles). The standard deviations over the strains are then calculatedfor a given number of cycles.

In FIG. 1, the first curve 15 (thin line) is (diagrammatically) the meancurve obtained for a steel produced according to the prior art. Thisfirst mean curve C-N is surrounded by two curves 16 and 14 in brokenthin lines. These curves 16 and 14 are respectively situated at adistance of +3σ₁ and −3σ₁ from the first curve 15, σ₁ being the standarddeviation of the distribution of the experimental points obtained duringthese fatigue tests, and ±3σ₁ statistically corresponds to a confidenceinterval of 99.7%. The distance between these two curves 14 and 16 shownin broken lines is therefore a measurement of the dispersion of theresults. The curve 14 is the limiting factor for the dimensions of apiece.

In FIG. 1, the second curve 25 (thick line) is (diagrammatically) themean curve obtained from the results of fatigue tests performed on thesteel produced according to the invention under a stress according toFIG. 2. This second mean curve C-N is surrounded by two curves 26 and 24in broken thick lines, respectively situated at a distance of +3σ₂ and−3σ₂ from the second curve 25, σ₂ being the standard deviation of thedistribution of the experimental points obtained during these fatiguetests. The curve 24 is the limiting factor for the dimensions of thepiece.

It will be noted that the second curve 25 is situated above the firstcurve 15, which means that under a fatigue stress at a strain levelC_(max), the steel test pieces produced according to the invention breakon average at a higher number N of cycles than that where the steel testpieces according to the prior art break.

Furthermore, the distance between the two curves 26 and 24 shown inbroken thick lines is smaller than the distance between the two curves16 and 14 shown in broken thin lines, which means that the fatiguebehavior dispersion of the steel produced according to the invention islower than that of the steel according to the prior art.

Thus, for a given strain, the curve 14 relative to a steel producedaccording to the prior art yields lower bench life values for a piecethan the curve 24 relative to a steel produced according to theinvention.

FIG. 1 illustrates the experimental results summarized in tables 1 and 2below.

Table 1 provides the results for a low-cycle fatigue stress according toFIG. 2 with a zero strain C_(min), at different temperatures: 20° C.,200° C., and 400° C. A low cycle fatigue means that the stress frequencyis in the vicinity of 1 Hz (the frequency being defined as the number ofperiods T per second).

It will be noted that for a given value of the number N of cycles, theaverage fatigue strain value necessary to break a steel according to theinvention is higher than the mean fatigue strain value M (set at 100%)necessary to break a steel according to the prior art. The dispersion(=6 σ) of the results at that number N of cycles for a steel accordingto the invention is lower than the dispersion of the results for a steelaccording to the prior art (dispersions expressed in percentage of themean value M).

TABLE 1 Low cycle fatigue Steel according to Steel produced accordingtest conditions the prior art to the invention N Temperature CDispersion C Dispersion 10⁵  20° C. 100% M 40% M 125% M 20% M 10⁵ 200°C. 100% M 30% M 137% M 15% M 3 · 10⁴ 400° C. 100% M 40% M 112% M 15% M

Table 2 provides the results for a vibrational fatigue stress, i.e. afrequency of approximately 80 Hz, at 200° C. The stress is identical tothat of FIG. 2 with a non-zero minimum strain C_(min) (fraction of themaximum strain C_(max)).

It will be noted that for a given value of the number N of cycles, themean fatigue strain value necessary to break a steel according to theinvention is higher than the mean fatigue strain value M necessary tobreak a steel according to the prior art. The dispersion of the resultsat that number N of cycles for a steel according to the invention islower than the dispersion of the results for a steel according to theprior art.

It will be noted that the minimum value C_(min) has very littleinfluence over the results.

TABLE 2 Steel produced Vibrational fatigue test Steel according to theaccording to the conditions prior art invention N Temperature C_(min) CDispersion C Dispersion 4 · 10⁶ 200° C. C_(max)/20 100% = M 30% M 120% M12% M 4 · 10⁵ 200° C. C_(max)/2 100% = M 30% M 126% M 14% M

The results of these tests therefore show that it is indeed thesecondary AlN precipitates in the steel according to the prior art,formed during the last heat shaping pass at a temperature above thesolid-state solubility temperature of those precipitates in the steel,that are responsible for the low fatigue bench life values of thatsteel.

The SEM (scanning electron microscope) observations done by theinventors on many steel test pieces according to the prior art andaccording to the invention corroborate these findings.

FIG. 3 is an SEM fractography of the fracture surface of a steel testpiece according to the prior art. A secondary precipitate can be seenthere. This precipitate assumes the form of a tab whereof thedimensions, indicated in the figure, are 17 μm and 22 μm for a thicknessof 0.4 μm. These secondary precipitates are present in significantquantities in the steel according to the prior art, and are practicallyabsent from steel according to the invention.

FIG. 4 is an SEM fractography of the fracture surface of the steel testpiece according to the invention. A primary precipitate can be seenthere. This precipitate has a substantially three-dimensional shape, andits dimensions, indicated in the figure, are of the same order ofmagnitude: 13 μm×8 μm×3 μm.

Advantageously, not only is the temperature of the last heat shapingpass lower than the solid-state solubility temperature of the aluminumnitrides in the steel, but the heat shaping temperature of each of theheat shaping passes before the last heat shaping pass is also lower thanthat solid-state solubility temperature.

Thus, substantially no harmful secondary precipitates form throughoutthe steel production method.

For example, the Al content of the steel is between 0.5% and 2%.

For example, in the steel, the C content is less than 0.4%, the Crcontent is between 0.5% and 7%, the Ni content is between 6% and 18%,and the Co content is between 4% and 18%.

A piece can be made from a martensitic steel according to the invention.For example, this piece is an engine transmission shaft, in particularfor an airplane engine.

The invention claimed is:
 1. A martensitic steel obtained by a methodwherein a heat shaping temperature of each heat shaping pass is lessthan a solid-state solubility temperature of aluminum nitrides in thesteel, the martensitic steel comprising: alloying elements such that thesteel can be hardened by an intermetallic compound and carbideprecipitation, with an Al content of between 0.4% and 3%, wherein anumber of aluminum nitrides per surface unit of the steel whereof ashape is such that a smallest dimension of the precipitate is at mostone twentieth of a largest dimension of the precipitate and that largestdimension is greater than 10 μm, is statistically less than 10⁻¹². 2.The martensitic steel according to claim 1, wherein the Al content ofthe steel is between 0.5% and 2%.
 3. The martensitic steel according toclaim 1, wherein, in the steel, the C content is less than 0.4%, the Crcontent is between 0.5% and 7%, the Ni content is between 6% and 18%,the Co content is between 4% and 18%.
 4. A mechanical component madefrom the martensitic steel according to claim
 1. 5. An airplane enginetransmission shaft made from the martensitic steel according to claim 1.6. The martensitic steel according to claim 1, wherein the solid-statesolubility temperature is 1025° C.